Three-dimensional imaging is dramatically expanding the ability of researchers to examine biological specimens, enabling a peek into their internal structures. And recent advances in X-ray diffraction methods have helped extend the limit of this approach.

While significant progress has been made in optical microscopy to break the diffraction barrier, such techniques rely on fluorescent labeling technologies, which prohibit the quantitative 3-D imaging of the entire contents of cells. Cryo-electron microscopy can image structures at a resolution of 3 to 5 nanometers, but this only works with thin or sectioned specimens.

And although X-ray protein crystallography is currently the primary method used for determining the 3-D structure of protein molecules, many biological specimens -- such as whole cells, cellular organelles, some viruses and many important protein molecules -- are difficult or impossible to crystallize, making their structures inaccessible. Overcoming these limitations requires the employment of different techniques.

Now, in a paper published May 31 in Proceedings of National Academy of Sciences, UCLA researchers and their collaborators demonstrate the use of a unique X-ray diffraction microscope that enabled them to reveal the internal structure of yeast spores. The team reports the quantitative 3-D imaging of a whole, unstained cell at a resolution of 50 to 60 nanometers using X-ray diffraction microscopy, also known as lensless imaging.

Researchers identified the 3-D morphology and structure of cellular organelles, including the cell wall, vacuole, endoplasmic reticulum, mitrochondria, granules and nucleolus. The work may open a door to identifying the individual protein molecules inside whole cells using labeling technologies.